This invention relates to a closed-circuit type respirator, and more particularly to a closed-circuit type respirator capable of preventing a risk of insufficient supply of oxygen in the respiratory system at static condition.
FIG. 2 shows the construction of a prior art closed-circuit respirator. In the respirator, the air from a mask a passes through an exhalation valve and an exhalation pipe b into a cleaning canister c where carbon dioxide in the exhaled air is absorbed, and the remaining air enters a respirator bag d and passes through an inhalation pipe and an inhalation valve e to enter the mask a again to be inhaled into the lung responding to each breathing. Oxygen in an amount corresponding to that consumed in the lung is automatically supplied into the inhalation pipe e from a pressurized oxygen container f via an automatic demand valve g. Since breathing becomes difficult due to insufficient oxygen in the respirator if the amount of oxygen consumed exceeds the amount supplied, a bypass valve h is provided in the respirator to arbitrarily supply oxygen by manual operation. On the other hand, if the amount of oxygen supplied exceeds the amount consumed, the pressure in the respirator increases and causes the user to feel pressed. Therefore an automatic relief valve i is provided to automatically release excessive air to restore the pressure to an optimum level. Thus in the prior art, a respirator is either of a type in which a bypass valve for oxygen is opened at the start of use to let a certain amount of oxygen directly into the circulation system to inflate a breathing bag to the fullest extent to thereby let out the air accumulated in the system, or of a type in which a constant flow of oxygen in a given amount is supplied in the circulation system together with the air flow caused by breathing to thereby fill the system with 100% oxygen in due time. Either of these types aims at constantly providing 100% oxygen for inhalation.
The applicant of the present invention, on the other hand, proposed in Utility Model Registration No. 1380741, patent application No. Sho 57-219488 and Utility Model application No. Sho 59-003128 to re-utilize as much air in the circulation system as possible without discharging it outside and to maintain oxygen concentration in the system approximately below 45%. In this way excess in oxygen amount can be avoided even if a closed-circuit respirator is used over a long period of time under atmospheric or higher pressure, and the portion of gas discharged during use will contain less than 45% of oxygen and over 55% of nitrogen instead of 100% oxygen, whereby a more economical device which is also effective in preventing any risks can be obtained.
However, in said system, the minimum oxygen concentration drops to the marginal value of 21% required for normal respiration and oxygen insufficiency to a certain extent may arise depending on the static condition.
Since an appropriate portable oximeter is not available at present, an oximeter is not generally attached to a closed-circuit respirator, and it is necessary to check against any risk of oxygen insufficiency which may occur under static condition as a safety precaution.